The present invention relates generally to wireless communication networks, and, more particularly, to a gain control system for a wireless communication system.
A wireless communication network includes a multiple wireless communication systems, such as base stations and user equipment devices (UEs). The base stations and UEs communicate using radio-frequency (RF) signals, which conform to specific standards and technologies including long term evolution (LTE), high speed packet access (HSPA) and third generation partnership project (3GPP) standards. RF signals used in LTE based wireless communication have multiple sub-frames. These sub-frames carry user-data used to transfer information between the base stations and UEs. The sub-frames can be of two types, user sub-frames and silent sub-frames. Sub-frames that include user-data are referred to as user sub-frames and sub-frames that do not include user-data, but may have interference from other RF signals, are referred to as silent sub-frames. User sub-frames are further categorized into user sub-frames with low user-data and user sub-frames with high user-data. User sub-frames that carry voice over Internet protocol (VoIP) data and require low resource allocation for transmission are referred to as user sub-frames with low user-data. Such user sub-frames are characterized by a low number of resource blocks (RBs). On the other hand, user sub-frames that carry full-data traffic and require high resource allocation for transmission are referred to as user sub-frames with high user-data. Such user sub-frames are characterized by a high number of RBs.
A typical wireless communication system (both a base station and a UE) includes an RF front-end, a gain controller, a gain stage, an analog-to-digital converter (ADC), and a processor, which may be referred to as to baseband processor. The processor executes a wireless protocol stack. The RF front-end includes an antenna that receives the RF signal and signal-conditioning circuits such as filters that perform conditioning operations on the received RF signal. The gain stage amplifies the RF signal. The ADC converts the amplified RF signal from the analog domain to digital baseband samples in the digital domain. One of the layers, typically the physical layer of the wireless protocol stack that runs on the baseband processor, receives and decodes the user-data of the user sub-frames of the digital baseband samples.
The gain stage includes an amplifier that amplifies a portion of the RF signal associated with a sub-frame based on a gain value provided by the gain controller. The gain controller includes a gain register that stores the gain value. The gain controller is configured to operate in two modes, automatic gain control (AGC) and manual gain control (MGC). In AGC mode, the gain controller computes and stores the gain value associated with a received sub-frame and uses the stored gain value for a subsequent sub-frame. In the MGC mode, the gain controller uses a gain value generated by the processor for the subsequent sub-frame. The amplified RF signal received by the ADC must be within a predetermined operating range that is specific to the ADC and refers to power levels of the RF signal at which the ADC functions optimally. To reach the predetermined operating range, the average RF signal power needs to be amplified by the gain stage to a pre-configured set-point. For example, if the ADC set-point is −5 dBm, an RF signal having an average power of −33 dBm must be amplified by the gain stage with a gain value of 28 dB to enable optimum conversion by the ADC. The ADC further has a saturation point associated with it. Typically, the value of the saturation point is 7 dBm. Thus, if the average power of the amplified RF signal crosses 7 dBm, the ADC may not be able to optimally convert and the wireless protocol stack may not be able to decode the digital baseband samples.
On certain occasions, the wireless communication system may receive one or more silent sub-frames between multiple user sub-frames, a scenario that is referred to as a burst traffic scenario. When the gain controller is configured to operate in AGC mode and silent sub-frames or user sub-frames with low user-data are received, the gain controller computes a first gain value to amplify a portion of the RF signal associated with the silent sub-frames or the user sub-frames with low user-data, to the operating range of the ADC. The first gain value is stored in the gain register and corresponds to the silent sub-frame or the user sub-frame with low user-data. Since a very low power is associated with the silent sub-frames or user sub-frames with low user-data, the first gain value is very high. For example, the portion of the RF signal associated with a silent sub-frame in LTE may have an average power of −33 dBm either due to interference in the RF environment. In another example, the portion of the RF signal associated with a user sub-frame with low user-data may have an average power of −33 dBm because it carries a low amount of data. Thus, the first gain value generated by the gain controller would be 28 dB. When a user sub-frame having high user-data is received subsequently, the gain controller provides the first gain value to the gain stage. If the portion of the RF signal associated with the user sub-frame having high user-data has an average power of −19 dBm, the RF signal is amplified to a power of 9 dBm, which is beyond the saturation point of the ADC. Therefore, the wireless protocol stack may not be able to decode this user-sub-frame. However, since the gain controller is operating in AGC mode, the gain controller adjusts the gain value of the next user sub-frames, thereby facilitating the decoding of the subsequent user sub-frames. Thus, decoding fails for the first user sub-frame having high user-data that follows the silent sub-frame or the user sub-frame with low user-data in the burst traffic scenario, which adversely affects the decoding performance of the wireless communication system.
One known technique to overcome this problem is to change the configuration of the RF front-end by tuning components of the RF front-end during the design stage. The gain controller has many parameters that allow control over the gain value provided for sub-frames. These parameters can be programmed or tuned to provide an optimum gain value. However, this technique does not improve the decoding performance of the wireless communication system in various traffic scenarios, such as burst traffic scenarios. Further, this technique requires multiple iterations to arrive at the optimum gain value and therefore increases the wireless communication system design time.
Therefore it would be advantageous to have a system for providing gain control in a wireless communication system that prevents an ADC of the wireless communication system from reaching saturation, and avoids decoding failures associated with user sub-frames.